1. Field of the Invention
The present invention relates to a technology for distributing key data for using content to devices with permission to use the content.
2. Description of the Related Art
To prevent the illegitimate use of digital content data (hereafter referred to as content) various technologies have been used. These technologies permit, for example, uses such as encryption and decryption of the content in legitimate devices, and prevent such uses in illegitimate devices. One of these technologies is laid open in Patent Document 1, which describes a technology for distributing the key data, in the manner described below, such that only legitimate devices can acquire the key data for using the content.
Each device using the content holds an individual key particular to itself. The key data for using the content is encrypted using the respective individual keys held by each of the legitimate devices, and distributed. Legitimate devices decrypt what they receive using their individual keys and obtain the key data, but illegitimate devices fail to decrypt what they receive, even if they have their own keys, and cannot obtain the key data. Hence, illegitimate use of the content can be prevented.
If, however, a device permitted to use of the content outsources the encryption or decryption processing to another device, the key distribution device cannot confirm whether or not the outsource destination device (i.e., the key data destination device) has the right to use the content. This is dangerous because there is a possibility that key data will be distributed to devices that do not have the right to use of the content.    [Patent Document 1] Japanese laid open patent application 2002-281013.    [Non-patent Document 1] “Gendai ango riron” (Modern encryption theory), IKENO Shinichi, KOYAMA Kenji, The Institute of Electronic, Information and Communication Engineers.    [Non-patent Document 2] “Ango riron nyumon” (An introduction to encryption theory)    OKAMOTO Eiji, Kyoritsu Publishing Inc.